Magnetic compensator



Jan. lO, 1956 T. A. MOLAUCHLAN 2,730f64l MAGNETIC COMPENSATOR Filed June1l, 1953 ff/Mm-X [noem:

Unite MAGNETIC COMPENSATUR Application .lune 11, 1953, Serial No.360,978

2 Claims. (Cl. 313-75) This invention relates to an improved compensatorfor changeable local magnetic fields.

My compensator is especially suited for electron microscopes, but it isapparent that it can be used with other mechanisms which are similarlydisturbed by changeable local elds. In the example of an electronmicroscope, an electron beam from a suitable source passes through anevacuated specimen chamber and impinges on a specimen at an angle ofincidence of approximately 90. Any magnetic field that acts on this beamtends to deflect it away from the specimen. Such microscopes commonlyare equipped with means which compensate for constant magnetic fieldsthat act on this beam, such as that of the earth. However, those withwhich l am familiar lack effective means to compensate for changeablefields, and it often has been necessary to resort to complete magneticshielding to overcome the effects of such fields.

An object of the present invention is to provide an improved andsimplified compensator which effectively overcomes the effects of localchangeable magnetic fields and which eliminates the need for elaborateshielding of the mechanism to which it is applied.

A further object is to provide an improved compensator which includesopposed hook-shaped arms of magnetic material arranged to have inducedtherein a magnetic field proportional to and opposite a disturbingfield, whereby the latter is effectively compensated.

In accomplishing these and other objects of the invention, I haveprovided improved details of structure, a preferred form of which isshown in the accompanying drawing, in which:

Figure 1 is a top plan view, partly in section, of a compensator whichembodies features of my invention; and

Figure 2 is a vertical sectional View on line lI-II of Figure l.

As an example of suitable environmental structure for the compensator ofmy invention, the drawing shows a portion of a cylindrical specimenchamber of an electron microscope. This chamber is formed of nonmagneticmaterial and is evacuated after insertion of a specimen indicated at X.An electron beam a--b from a source Y beyond the opposite end of thechamber passes therethrough along the central axis and impinges on thespecimen. Any local changeable field, such as that indi` cated in Figurel, tends to defiect this beam away from the specimen and thus preventsproper observation thereof.

The compensator of my invention comprises a pair of opposed hook-shapedarms 12 and 12a which are of a soft magnetic material and are of likeconstruction. Each arm has a screw-threaded shank 13 and a reverselybent tip 14, which are axially aligned. The two arms are held togetheras a rigid assembly by pairs of clamping bars 15 of a conductivenonmagnetic material, such as aluminum or brass, which does not acquirean electrostatic charge when exposed to an electron beam. In theirclamped position the Shanks and tips 13 and 14 of both arms are all inalignment, and the two tips 14 are separated by a gap 16.

Zfiil iatented Jan. 10, 1956 The assembly of the two arms 12 and 12a andthe clamping bars 15 is mounted inside the chamber 10 with the commonaxis of the Shanks and tips extending across the diameter of the chamberparallel to the direction of the external changeable field. The mountingmeans illustrated includes spacers 17 which fit against the inside wallof the chamber and nuts 18 which are threadedly engaged with the Shanksand abut said spacers. The spacers and nuts preferably are formed of thesame material as the clamping bars 15. The electron beam a-b passesthrough the gap 16 between the two tips 14.

A pair of bushings 19 and 19a are xed to the exterior of the chamber 10,for example by soldering or adhesively. The bushing 19 is attachedopposite the shank of the arm 12 and the bushing 19a opposite that ofthe arm 12a. Plugs 2) and 20a are threadedly engaged with the bushings19 and 19a respectively for adjustment toward and away from the Shanksof the arms. These plugs preferably are of similar material to the armsand are axially aligned with the Shanks and tips thereof.

The changeable local field always induces a proportional eld in the arms12 and 12a. The induced field is of course much more intense than theexternal eld, but it acts on the electron beam only in the relativelysmall region of the gap 16. The exact relation between the intensitiesof the two fields can be adjusted by moving the plugs 2i)l and 20acloser to or farther from the ends of the arms 12 and 12a. The reversebends in these arms causes the field induced therein to act in adirection opposite that of the external field and consequentlycompensates for the latter.

From the foregoing description it s seen that the present inventionaffords a compensator of very simple construction that effectivelyovercomes disturbances caused by local changeable magnetic fields. Iteliminates any need for the more expensive and cumbersome magneticshielding.

While I have shown and described only a single embodiment of theinvention, it is apparent that modifications may arise. Therefore I donot wish to be limited to the disclosure set forth but only by the scopeof the appended claims.

I claim:

l. 'I'he combination, with a device which includes a cylindrical chamberand means for passing an electron beam through said chamber along itscentral axis, of a compensator for changeable local magnetic fields of aknown direction comprising a pair of hook-shaped arms of soft magneticmaterial each having a shank and a reversely bent tip aligned with itsshank, non-magnetic and conductive means clamping said arms togetherwith their tips separated to form a gap and the Shanks and tips of thetwo arms all in alignment, and means mounting said arms inside saidchamber with said Shanks and tips lying along a diameter of the chamberin the direction of the changeable local field and with the central axisof the chamber passing through said gap.

2. A combination as defined in claim 1 including mounting means on theexterior of said chamber opposite the extremities of said Shanks, andplugs of soft magnetic material adjustably supported in said exteriormounting means in alignment with said Shanks and tips for Varying theintensity of magnetic field induced in said arms.

References Cited inthe file of this patent UNITED STATES PATENTS2,153,643 Roosenstein et al Apr. l1, 1939 2,425,125 Richards Aug. 5,1947 2,637,000 Page Apr. 28, 1953 2,651,000 Linder Sept. 1, 1953

